The present invention relates to non-thermal plasma reactors and more particularly to a dielectric barrier discharge system for decomposing hazardous compounds in a liquid or a gas.
Certain compounds including sulfur oxides, nitrogen oxides and carbon monoxide in power plant flue gases must be controlled to meet stringent government emission regulations. These compounds are either toxic or are precursors to acid rain deposition and photochemical smog. The industry has devoted considerable effort to develop a variety of technologies to reduce the pollutant emissions from the exhaust stream of combustion processes. However these technologies have substantial disadvantages and more effective and economical measures are needed.
Plasma is regarded as the fourth state of matter (ionized state of matter). Unlike thermal plasmas, non-thermal plasmas (NTPs) are in gaseous media at near-ambient temperature and pressure but have electron mean energies considerably higher than other gaseous species in the ambient environment. NTP species include electrically neutral gas molecules, charged particles in the form of positive ions, negative ions, free radicals and electrons, and quanta of electromagnetic radiation (photons). These NTP species are highly reactive and can convert hazardous gases to non-hazardous or less hazardous and easily-managed compounds through various chemical reaction mechanisms. In contrast to thermal processes (such as thermal plasma), an NTP process directs electrical energy to induce favorable gas chemical reactions, rather than using the energy to heat the gas. Therefore, NTP is much more energy-efficient than thermal plasma.
NTPs can be generated by electric discharge in the gas or injection of electrons into the gas by an electron beam. Electron beams must be accelerated under a high vacuum and then transferred through special windows to the reaction site. The reaction site must be sized with respect to the penetration depth of the electrons. It is much more difficult to scale-up the size of an electron beam reactor than an electric discharge reactors. Therefore, electron beam reactors are less favored than electric discharge reactors.
Among the various types of electric discharge reactors, pulse corona and dielectric barrier (silent) discharge reactors are very popular for their effectiveness and efficiency. However, pulse corona reactors have the major disadvantage of requiring special pulsed power supplies to initiate and terminate the pulsed corona. Consequently, dielectric barrier discharge has become a fast growing technology for pollution control.
Cylindrical and planar reactors are two common dielectric barrier discharge reactor configurations. Both of these configurations are characterized by the presence of one or more insulating layers in a current path between two metal electrodes, in addition to the discharge space. Other dielectric barrier discharge reactors include packed-bed discharge reactors, glow discharge reactors, and surface discharge reactors.
There are several major difficulties in the practical use of dielectric barrier discharge reactors for hazardous gas removal. These difficulties include an expensive power supply, a low energy efficiency and flow rate, and the blocking of discharge volume by dusts in the feed gas and/or solid mineral compounds produced during the plasma reactions. More effective and economical dielectric barrier discharge reactors are desired.
One aspect of the present invention relates to a dielectric barrier discharge system which includes first and second non-thermal plasma reactors that are coupled together in series. The first reactor includes a first surface discharge electrode which defines a first discharge path along the first surface discharge electrode. The second reactor includes second and third electrodes which are separated by a gap and define a second discharge path which extends across the gap.
Another aspect of the present invention relates to a dielectric barrier discharge system for treating a fluid comprising dust, sulphur oxide and nitrogen oxide. The system includes a pretreatment non-thermal plasma reactor and a main non-thermal plasma reactor. The pretreatment non-thermal plasma reactor has a surface discharge electrode for producing a surface plasma in the fluid along the surface discharge electrode. The surface plasma removes a first portion of the dust and decomposes a first portion of the sulphur oxide and nitrogen oxide. The main non-thermal plasma reactor is coupled to the pretreatment non-thermal plasma reactor and has oppositely polarized electrodes which are separated by a gap. The oppositely polarized electrodes produce a plasma in the fluid across the gap which removes a second portion of the dust and decomposes a second portion of the sulphur oxide and nitrogen oxide.
Another aspect of the present invention relates to a method of decomposing a compound in a fluid. The method includes: passing the fluid along a first surface discharge electrode in a pretreatment non-thermal plasma reactor; electrically exciting the first surface discharge electrode to generate a first surface plasma in the fluid along the first surface discharge electrode; passing the fluid through a gap between second and third oppositely polarized electrodes in a main non-thermal plasma reactor; and electrically exciting the second and third oppositely polarized electrodes to generate a second plasma in the fluid across the gap.